Group III nitride semiconductors offer a direct transition over a band gap energy from visible light to ultraviolet rays, and excel in the light emission efficiency, and thus have been manufactured as semiconductor light-emitting devices such as a light emitting diode (LED) and a laser diode (LD) for use in various applications. In addition, when used for an electronic device, Group III nitride semiconductors have a potential to provide electronic devices having characteristics superior to those using conventional Group III-V compound semiconductors.
Such Group III nitride compound semiconductors are, in general, produced from trimethyl gallium, trimethyl aluminum, and ammonia as raw materials through a Metal Organic Chemical Vapor Deposition (MOCVD) method. The MOCVD method is a method in which a vapor of a raw material is introduced into a carrier gas to convey the vapor to the surface of a substrate and decompose the raw material on the surface of the heated substrate, to thereby grow a crystal.
Hitherto, a single crystal wafer of a Group III nitride semiconductor has not been commercially available, and Group III nitride semiconductors are, in general, produced by growing a crystal on a single crystal wafer of a different material. There is a large lattice mismatching between such a different kind of substrate and a Group III nitride semiconductor crystal to be epitaxially grown thereon. For example, when gallium nitride (GaN) is grown on a sapphire (Al2O3) substrate, there is a lattice mismatching of 16% therebetween, and when gallium nitride is grown on a SiC substrate, there is a lattice mismatching of 6% therebetween. In general, a large lattice mismatching as in the above leads to a problem in that it is difficult to epitaxially grow a crystal directly on a substrate, or a crystal, even if grown, can not gain excellent crystallinity.
Thus, for epitaxially growing a Group III nitride semiconductor crystal on a single crystal sapphire substrate or a single crystal SiC substrate through a Metal Organic Chemical Vapor Deposition (MOCVD) method, a method has been proposed and generally performed in which, firstly, a layer called a low temperature buffer layer made of aluminum nitride (AlN) or aluminum nitride gallium (AlGaN) is laminated on a substrate, and then a Group III nitride semiconductor crystal is epitaxially grown thereon at a high temperature.
In addition, regarding a method in which a layer of such as AlN is formed as a barrier layer on a substrate by a method other than the MOCVD method, and another layer is formed thereon by a MOCVD method, a method has been proposed in which a buffer layer is formed by high frequency sputtering, and a crystal having the same composition is grown thereon by a MOCVD method (for example, Patent Document 1).
However, the method disclosed in Patent Document 1 has a problem in that an excellent crystal cannot be stably produced.
Thus, in order to stably produce an excellent crystal, for example, there have been proposed a method for annealing a buffer layer in a mixed gas made of ammonia and hydrogen on completion of its growth (for example Patent Document 2), and a method for forming a buffer layer by DC sputtering at a temperature of 400° C. or higher (for example Patent document 3).
On the other hand, research has been conducted on the manufacture of a Group III nitride semiconductor crystal by sputtering. For example, with a purpose of laminating high resistance GaN, a method for forming a GaN film directly on a substrate made of sapphire by a sputtering method has been proposed (for example, Patent Document 4).
In addition, there is a method in which reverse sputtering is conducted with use of Ar gas as a pretreatment on a semiconductor layer for forming an electrode on the semiconductor layer (for example, Patent Document 5). According to the method described in Patent Document 5, the electrical contact resistance characteristic between the semiconductor layer and the electrode can be improved by applying reverse sputtering to the surface of the Group III nitride compound semiconductor layer.
For forming a film of GaN which is doped with Mg as an acceptor impurity on a substrate with use of a sputtering method as mentioned above, hydrogen (H2) circulation into the atmosphere gas in a chamber makes it possible to form a GaN film with excellent crystallinity. However, hydrogen atoms (H) constituting the hydrogen gas are easily bonded with Mg, which brings about a problem regarding reduction in the dopant (Mg) carrier concentration.
Moreover, when a GaN crystal film is formed by a MOCVD method, ammonia gas (NH3) is generally used as an N (nitrogen) source. Therefore, in this case, hydrogen is mixed into the crystal film. Thus, a method has been employed in which hydrogen emission from the GaN crystal is induced by a method called an activation annealing process which performs an annealing treatment at a temperature of 600° C. or higher in an inert gas. However, in the structure of a Group III nitride semiconductor light-emitting device such as an LED, if annealing is performed, the annealing heat brings about a problem of damaging the light-emitting layer made of an InGaN crystal. Moreover, normally, even if the GaN crystal film is annealed, about 1/10 to ⅕ of the doped Mg remains unactivated, which brings about a problem regarding difficulty in providing the annealing effect.
Patent Document 1: Japanese Examined Patent Application, Second Publication No. Hei 5-86646
Patent Document 2: Japanese Patent No. 3440873
Patent Document 3: Japanese Patent No. 3700492
Patent Document 4: Japanese Unexamined Patent Application, First Publication No. Sho 60-039819
Patent Document 5: Japanese Unexamined Patent Application, First Publication No. Hei 8-264478